1. Field of the Invention
The present invention relates to selected trinuclear novolak oligomers useful as backbones for certain photoactive compounds. Further, the present invention relates to such photoactive compounds formed by the esterification of these trinuclear novolak oligomers with sulfonyl halides of o-naphthoquinone diazides. Still further, the present invention also relates to radiation sensitive mixtures (e.g. those particularly useful as positive-working photoresists) containing the combination of these photoactive compounds with alkali-soluble binder resins dissolved in a solvent. And furthermore, the present invention also relates to substrates coated with these radiation sensitive mixtures as well as the process of coating, imaging and developing these radiation sensitive mixtures on these substrates.
2. Description of Related Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. In these processes, a thin coating or film of a photoresist composition is generally first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, ion beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes.
After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some processes, it is desirable to bake the imaged resist coating before this developing step. This intermediate step is sometimes called post-exposure bake or PEB.
There are two types of photoresist compositions--negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g. the Wolff rearrangement reaction of the photactive compound occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate are protected where the photoresist coating still remains and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
End users of photoresists are demanding photoresist formulations which possess better lithographic properties for the fabrication of smaller microelectronic circuits. The lithographic properties which are critical to these end-users include the following: (1) good resolution capabilities in both the micron and submicron ranges without incomplete development in the exposed areas (i.e. scumming); (2) higher thermal image deformation temperatures (e.g. above 120.degree. C.); (3) relatively fast photospeeds; (4) good adhesion to substrate; (5) good developer dissolution rates; (6) good process latitute; (7) near to absolute vertical profiles (or good contrast) between exposed and unexposed photoresist areas after development; (8) good resistance to etching solutions and plasma etching techniques; (9) reduced tendency to form insoluble particulates and (10) mask linearity.
Generally, in the past efforts to improve one of these lithographic properties have caused significant decreases in one or more of the other lithographic properties of the photoresist. Accordingly, there is a need for improved photoresist formulations which possess all of these desired properties. The present invention is believed to be an answer to that need.
In the past, reactions between 2,6-bis(hydroxymethyl)-p-cresol and alkyl monohydric phenols to form novolak oligomers have been known. These novolak oligomers were then reacted with 6-diazo-5,6-dihydro-5-oxo-1-naphthalene-1-sulfonic acid to form a photoactive compound and then used to make a positive-working photoresist. See Japanese Patent Publication (Kokai) No. 62-10646(A) assigned to Kanto Chemical Company and published on Jan. 19, 1987. One of the novolak oligomers taught by this reference has the following formula (PA-I): ##STR2## Separately, reaction between 2,6-bis(hydroxymethyl)-p-cresol and resorcinol to form oligomeric polyhydric phenol novolaks were then reacted with epihalohydrin or the like to form a polyglycidic ether which was combined with curing agent to form a curable epoxy resin composition. See U.S. Pat. No. 4,614,826, which issued to Katayama et al on Sept. 30, 1986. The reaction product of 2,6-bis(hydroxymethyl)-p-cresol and resorcinol according to this reference has the following chemical formula (PA II): ##STR3##